1. Place potatoes and garlic cloves in a sauce pan with water at least 1" above potatoes. Salt and bring to a boil. Once the potatoes are soft enough to stick a fork into and they break apart, remove from heat and strain the water. In the sauce pan or a separate bowl, add the boiled potatoes, vegan butter, milk, and chives. Mash to a smooth consistency.
2. While potatoes are boiling, start to prepare the filling. Heat a skillet with the olive oil. When the oil is hot, add the onions and pinch salt; sweat until the onions are translucent. Next, crumble the seitan into the skillet and brown. Add the peas and carrots to the skillet and cook until tender. Season, to taste.
3. While the vegetables are cooking, preheat the oven to 375° F. In a cup, mix the cornstarch and cream of mushroom soup until the cornstarch is evenly mixed. Add to the seitan mixture and stir until even coated. Cook for just a few minutes until the soup has thickened and remove from the heat.
4. Using a spoon, start stuffing the mushrooms with the seitan mixture. Be sure to pack the mixture in so that all the gaps are filled. Top the mushrooms with the mashed potatoes liberally; you are basically sealing the stuffing in. Sprinkle the parmesan cheese over the top and place in the oven to 10 minutes on the top rack.
5. After 10 minutes, place the oven on broil and let the tops brown, about 3-5 minutes. Remove from the oven and serve.
Source of recipe: I wrote the recipe. For more information about this recipe, please go to my blog here: http://veganmiss.blogspot.com/2010/11/shephards-pie-stuffed-portobella.html